This invention relates to a method of preparing graphite fluoride by an eterogeneous contact reaction between solid carbon and fluorine gas, and more particularly to an improvement in the safety and reproducibility of the reaction.
Graphite fluoride or poly(carbon monofluoride) having the chemical structure expressed by (CF).sub.n, is a stable solid compound which can be prepared by a heterogeneous contact reaction between carbon and fluorine gas. As is known this compound is very low in surface energy and has been valued in various industries as a superior material for lubricants, water- and oil-repellents, anti-wetting agents, anti-contamination agents, activating agents for certain electrolytic cells, etc. Recently graphite fluoride of another type, i.e., poly(dicarbon monofluoride), expressed by (C.sub.2 F).sub.n, has also acquired great and increasing interest for generally similar aspects. Since the demand for such graphite fluorides is rapidly increasing, it has become a matter of importance and urgency to develop an industrially practicable and favorable method for large-scale production of graphite fluorides.
Generally the aforementioned reaction to form graphite fluoride (CF.sub.x).sub.n, where 0&lt;x&lt;1.3, is expressed by the following equation. ##EQU1## In practice, however, some side reactions take place as the reaction of Equation (1) proceeds. The side reactions are represented by the following equations: EQU C+F.sub.2 .fwdarw.CF.sub.4, C.sub.2 F.sub.6, etc. (2) EQU (CF.sub.x).sub.n .fwdarw.C+CF.sub.4, C.sub.2 F.sub.6, etc. (3)
The reaction of Equation (3) is the decomposition of the graphite fluoride formed by the intended reaction (1). Since the reactions (1) and (2) are both exothermic, the cause of the reaction (3) has been presumed to be the accumulation of the heat of formation of graphite fluoride by the reaction (1), augmented by the heat of the reaction (2), in the mixture of the formed graphite fluoride and the unreacted portion of the carbon material, and the consequential rise in the temperature of the mixture beyond the decomposition temperature of the graphite fluoride. In many cases, the decomposition reaction (3) is greatly promoted in a very short time with generation of a large amount of heat and large amounts of gaseous fluorocarbons, and results in explodingly rapid and violent decomposition of the entire graphite fluoride in the reaction system. In such cases it is impossible to obtain the intended product, graphite fluoride. Moreover, sometimes the reaction vessel is significantly damaged by the explosive decomposition of the graphite fluoride.
On the aforementioned presumption, it has been preventing the decomposition reaction of Equation (3) by preventing accumulation of heat in the solid phase of the reaction system according to Equation (1) has been tried. The proposals made heretofore in this regard include dilution of fluorine gas for the reaction of Equation (1) with a certain gas, e.g. CF.sub.4, to lower the rate of the reaction (1) and suppress the side reaction (3), limitation of the reaction temperature, and the use of a multi-stage fluorination apparatus.
However, we have recognized that almost instantaneous decomposition of the formed graphite fluoride often takes place even when the heat of reaction is effectively dissipated so that the temperature of the solid phase of the reaction system exhibits little rise.